It has been known for many years that a number of elements, alloys, and compounds when cooled below a critical temperature, the superconducting transition temperature T.sub.c, enter into a zero electrical resistance state. This state of matter is one of the most interesting and potentially important states that occurs in nature. When matter is in the superconducting state it exhibits a variety of both classical and quantum mechanical phenomena. Of greatest technical interest is the ability of a superconductor to carry large amounts of current without energy loss and thereby generate large magnetic fields at less power than conventional conductors such as copper.
A superconducting material is characterized by its superconducting transition temperature (T.sub.c --below which the material is superconducting and above which it is a normal conductor), upper critical magnetic field (H.sub.c2 --below which the material is in the superconducting state and above which it is a normal conductor), and critical current (J.sub.c --below which it is a superconductor and above which it is normal conductor). Of these, the T.sub.c and H.sub.c2 are dependent on the actual superconducting element, alloy, or compound while the critical current depends on both the superconducting element, alloy, or compound and the form of the material (single phase, multi-phase, composite structure). The dependence of the critical current on the microstructure of the material results from the need to pin the magnetic flux vortices that form in superconductors in the presence of a magnetic field. The phenomena is described in Wilson, M. N. "Practical Superconducting Materials", Superconductor Materials Science, Foner, S. and Schwartz, B. ed., Plenum Press (1981), pp. 63-131.
Important applications for superconducting materials include high field magnets, magnetic separation, magnetic levitation, electrical power transmission, electric motors, electrical generation, energy storage devices, and potentially, magnetically confined nuclear fusion. One of the major drawbacks to the widespread use of superconductors has been that the transition temperature, T.sub.c, has been limited to about 20 K.
A new class of superconductors known as high temperature oxide superconductors have recently been discovered, in which the transition temperature is generally above about 40 K., and in some above 77 K., the boiling point of liquid nitrogen. The basic structure and components of these perovskite materials have been known since 1980 and are described in Er-Rakho, L. et al. "A Series of Oxygen-Defect Perovskites Containing Cu.sup.II and Cu.sup.III ", Journal of Solid State Chemistry, 37 (1981) pp. 151-156, but their superconducting characteristics have only recently been discovered. The materials are oxygen-defective perovskites containing both Cu.sup.2+ and Cu.sup.3+ in the same lattice, generally comprising rare earth-copper-alkaline earth-oxygen. Specific examples of the new class of superconductors include but are not limited to Y.sub.1 Cu.sub.2 Ba.sub.3 O.sub.x, Ba.sub.0.15 La.sub.1.85 CuO.sub.x, Y.sub.0.5 Eu.sub.0.5 Cu.sub.2 Ba.sub.3 O.sub.x, Sr.sub.0.15 La.sub.1.85 CuO.sub.x and Er.sub.0.2 Y.sub.0.8 Cu.sub.2 Ba.sub.3 Oa.sub.x. Others are listed in Table I, below.
The oxide materials have such high transition temperatures and high critical magnetic fields that they may lead to a revolutionary change in a number of industries, primarily those dealing with energy, electrical equipment, and transportation. However, a serious problem exists concerning these new materials in that being oxides, they are characteristically brittle and difficult, if not impossible, to form directly into useful components such as wires, ribbons, tapes, fibers or composites for fabrication of superconducting devices.
The requirements for using superconductors in a practical manner are summarized in Millman "Fabrication Technology of Superconducting Material," Superconductor Materials Science, Foner, S. and Schwartz, B. ed., Plenum Press (1981), pp. 275-388. These can be divided into (i) intrinsic properties of the superconducting phases and (ii) mechanical/physical properties that would allow for manufacturing of superconducting devices. The intrinsic properties that are important are the T.sub.c, H.sub.c2, and J.sub.c. The mechanical/physical properties that are essential are: high strength and ductility, ability to make continuous wire/filaments/tapes, and good thermal conductivity.
A variety of methods have formerly been developed that allow the practical use of superconducting materials. For many applications, such as electromagnets, the desired form of the structural material used is a wire with sufficient mechanical strength and length to be susceptible to being wound into a tight coil. Superconductors do not generally possess the characteristic mechanical properties to be directly used in such applications and thus different solutions to this problem have been devised depending on the type of superconductor that is being used.
In the past, two types of materials were developed to meet the demanding requirements for use in high current, high magnetic field devices. The first materials were solid solution alloys such as NbTi (having T.sub.c equal to 8-10 K., and H.sub.c2 equal to 14-15 tesla at 4.2 K.) in which the Nb and Ti atoms form a random body-centered cubic lattice. These materials have limited ductility and as described by Millman, supra, can be co-extruded with copper to form useful superconducting wires. The second materials were the A-15 structure materials such as Nb3Sn (having T.sub.c equal to 18.1 K. and H.sub.c2 equal to 22.0 tesla at 4.2 K.). These materials are intrinsically brittle and their use has been primarily in the form of wires consisting of thin filaments of Nb.sub.3 Sn in a copper matrix, Millman, supra. In both of these examples the cooper provides mechanical strength, ductility, and thermal conductivity to the wire.
A number of other metallic superconducting materials are known such as Nb.sub.3 Al, (Hf-Zr)V.sub.2 and NbN, but are not commercially used today becuase no practical solution to the processibility problems mentioned above has been discovered. One proposal to solve mechanical processibility problems in the Hf-Zr-V alloy system has been to utilize a three step process, Tenhover, M., IEEE Transaction on Magnetics, Vol. Mag 17, No. 1, January 1981, pp. 1021-1024. Tenhover proposed to (i) melt spin a Hf-Zr-V alloy to form an amorphous metal alloy in the shape of a continuous ribbon, (ii) wind a coil to make an electromagnetic solenoid, and (iii) crystallize the amorphous Hf-Zr-V alloy to form the C-15 type superconducting compound (Hf-Zr)V.sub.2.
A process for the production of high temperature oxide superconductors in a manner to compensate for their brittleness is also required.